Oxford Botanic Garden

Wilsham Consulting Director – Stephen Gibson shares his design experience at the Oxford Botanic Garden.

I had great pleasure in designing a rainwater harvesting system for the Oxford Botanic Garden . The Oxford Botanic Gardens are world famous and one of the greatest locations in Oxford to visit.

 Water and especially rainwater, is especially precious for the gardens and has been for hundreds of years. The gardens conservatories have numerous simple brick or lead chambers for collecting rainwater and pumps such that the water can be used for watering.

A new staff building was to be designed and this was an ideal opportunity to capture addition rainwater from the roofs and additional hardstanding. The site did have challenging elements. Access was restricted and at least part of the gardens are in the River Cherwell flood plain. The groundwater level was therefore high and dewatering was not an acceptable environmental option. The Client wished to maximise storage of water so far as reasonably possible. A traditional chamber harvesting system was not viable.

I therefore designed a bespoke attenuation crate system, which was then rapped with a special impermeable liner. Joining such liners is a specialist skill in which overlapping sections are joined with very hot compacting rollers. An overflow was needed, which had a non return valve fitted to avoid backflow from the River Cherwell contaminating the rainwater at times of flooding. The attenuation was split into 2 sections; one taking flows only from the roof and the other taking flows from the external areas.

A specially design filtration system; based on bespoke French drain arrangement, of gravels, geotextiles and sands was designed for the flows from the external areas. A considerable amount of research into the use of external flows within rainwater harvesting systems at other Botanical Gardens around the world. It is understood to be one of the first Botanic Gardens in the world to use such flows. The Oxford Botanic Garden's Team said their experience was that rainwater was better, than tap water for many plants.

A performance specification was produced for the pumping system, such that with the inclusion of pressure switches and a 50 litre pressure vessel, the gardens could be provided with flows to the hoses similar to the pressure experienced with a mains connection. The pumping system manufacturer was TT Pumps.

We are keen to assist other Botanical Gardens around the World with their proposals. At Wilsham, we share the same values in water quality. Harvesting water is not just about being able to grow plants better, or reducing your water bill, but about accepting responsibility towards sustainable design.

Rainwater must be seen as an asset, not something to "be got rid of as quickly as possible".


Photos from the Oxford Botanic Gardens.